In wireless communication systems, the use of antenna arrays at the base station has been shown to increase both range, through increased gain, and capacity, through interference suppression. With adaptive antenna arrays, the signals received by multiple antenna elements are weighted and combined to improve system performance, e.g., by maximizing the desired receive signal power and/or suppressing interference. The performance of an adaptive antenna array increases dramatically with the number of antennas. Referring to an article entitled, "The Impact of Antenna Diversity on the Capacity of Wireless Communication Systems," by J. H. Winters, R. D. Gitlin and J. Salz, in IEEE Trans. on Communications, April 1994, it is shown that using an M element antenna array with optimum combining of the received signals can eliminate N.ltoreq.M-1 interferers and achieve an M-N fold diversity gain against multipath fading, resulting in increased range.
Most base stations today, however, utilize only two receive antennas with suboptimum processing, e.g., selection diversity where the antenna having the larger signal power is selected for reception and processing. It is desirable to be able to modify existing base stations to accommodate larger arrays of antennas and/or improved received signal combining techniques. However, modifying existing equipment is difficult, time consuming, and costly, in particular since equipment currently in the field is from a variety of vendors.
One alternative is to utilize an applique, which is an outboard signal processing box, interposed between the current base antennas and the input to the base station, which adaptively weights and combines the received signals fed to the base station, optionally utilizing additional antennas. FIG. 1 shows a base station utilizing an applique. A key to the viability of utilizing the applique approach is that it should require little, if any, modification of the base station equipment. This constraint implies that the processing performed by the applique must be transparent to the existing equipment. Ideally, the signal emerging from the applique should appear to the existing base station as a high-quality received signal from a single antenna.
As in many passband digital communications systems which operate at RF frequencies, it is often preferable to demodulate or "downconvert" the frequency band of interest to a lower frequency band and to perform the requisite digital signal processing on this lower frequency replica of the received signal. Downconversion permits the analog to digital (A/D) conversion and subsequent signal processing to be performed at a lower rate, thus decreasing the implementation cost and complexity of the digital signal processing hardware. Illustratively, downconversion to baseband will be considered.
The process of downconversion to baseband requires synthesis, at the receiver, of a reference signal located at the nominal carrier frequency of the signal to be demodulated. This reference signal is produced by a local oscillator (LO). At the relatively high RF frequencies used in cellular and Personal Communication System (PCS) band radio, practical considerations make it difficult to generate the LO signal with extreme precision, and some non-trivial frequency error or "carrier frequency offset" typically exists. Carrier frequency offset can result in performance degradation, especially when a coherent reference signal is used to derive the adaptive weights. In some circumstances, frequency offset alone can result in a significant excess mean-squared error (MSE). Therefore, there exists a need to reduce the performance degradation caused by carrier frequency offset.